An exposure apparatus to transfer a reticle fine pattern onto a wafer coated with a photosensitive material is employed in a photolithography process in semiconductor device manufacturing, or the like. In this sort of an exposure apparatus, a step-and-repeat method is mainly used for sequentially moving wafers into an exposure field of the exposure projection optical system and sequentially exposing the wafers to a reticle pattern.
In this step-and-repeat type projection exposure apparatus, to bring the exposure field on the wafer into correspondence with an image formation surface of the projection optical system, a focus/leveling sensor and a focus/leveling driving mechanism are provided. The focus driving is driving of a micromotion stage so as to bring a wafer surface position in an optical axis direction (z-axis direction) into correspondence with a focus position. Further, the leveling driving is detecting the inclination angle of the wafer surface and driving the micromotion stage to set the wafer surface in a horizontal position. The focus/leveling sensor measures a focus/leveling shift of the wafer, then the focus/leveling driving mechanism performs correction by the measured shift amount, and exposure is performed. In the case of a step-and-repeat method, as exposure is performed after the stoppage of a wafer, the amount of the focus/leveling driving does not influence the exposure accuracy.
On the other hand, there is an increasing need for expansion of a field of exposure area in accordance with upsizing of a semiconductor device in recent years. For this reason, a step-and-scan method meeting the requirement is increasingly employed. In the step-and-scan method, slit or circular-arc illumination light is emitted on a predetermined area of a reticle, then exposure is performed at a reduction ratio of a projection optical system in a slit lengthwise direction, while the reticle and the wafer are scanned in synchronization at the reduction ratio of a projection optical system in a widthwise direction of the slit.
In the step-and-scan method, a focus/leveling sensor and a focus/leveling driving mechanism are provided to bring the exposure field on the wafer into correspondence with the image formation surface of the projection optical system as in the case of the aforementioned step-and-repeat method. However, in the step-and-scan method, the slit (or circular-arc) exposure area on the wafer must be sequentially brought into correspondence with the image formation surface during scanning. For this purpose, in this method, a so-called look-ahead method is employed. The look-ahead method is measuring a focus/leveling shift by the focus/leveling sensor on the front side of the slit exposure area, i.e., on the front side in the scanning direction, and performing the focus/leveling driving based on the result of the measurement, thereby bringing the wafer surface in the measured point into the image formation surface of the projection optical system before the measured point comes to the slit exposure area (see, for example, Japanese Patent Application Laid-Open No. 10-097987).
In the look-ahead method, the amount of focus/leveling driving is calculated based on a discretely obtained measurement value. However, as a wafer has its own local distortion, a step portion due to suction by a wafer chuck to hold the wafer, a step portion caused by a foreign particle, or the like, in addition to a step portion caused by a formed pattern (process step), the driving amount calculated by the look-ahead operation is a large value in some cases. In such a case wherein steps different from the process step are to be followed, the following accuracy of focus/leveling is degraded, and further, the synchronization accuracy between the reticle and wafer is degraded. That is, in the step-and-scan method, different from the step-and-repeat method, as measurement and driving for focus/leveling are repeated during scanning, the amount of focus/leveling driving influences the exposure accuracy.
In the conventional art, a unique upper limit value is provided for the discretely obtained focus/leveling driving amount for improvement in the focus/leveling following accuracy and the synchronization accuracy. However, the focus/leveling following accuracy and the synchronization accuracy to the focus/leveling driving amount are low when the scanning speed or scan acceleration of the wafer stage is high, while the focus/leveling following accuracy and the synchronization accuracy are high when the scanning speed or scan acceleration of the wafer stage is low. In the above conventional art, the upper limit value to the focus/leveling driving amount is a unique value, and the upper limit value is adapted to a worst case wherein the scanning speed or scan acceleration is high. Accordingly, when the scanning speed or scan acceleration is low, the limit value is an excessive control value to the driving amount. For example, at a low scanning speed or scan acceleration, if the upper limit value of the focus/leveling driving amount is high, the following is possible and sufficient focus accuracy can be maintained regarding a wafer with large process steps. However, the above-described excessive control may hinder the focus/leveling following. As a result, the focus/leveling following accuracy may be degraded.